The silicon photomultiplier (SPM) is a semiconductor detector which operates in a manner which is similar to the optical and electrical operation of the vacuum tube based photomultiplier tube, or PMT or the microchannel plate, or MCP. The SPM is designed so that the detector converts incident photons into charge, multiplies that charge through an internal gain mechanism and outputs the resulting charge to the output of the SPM detector. In this way photons are converted into currents which can be easily measured using external circuitry.
The basic SPM is a large array of microcells. This is known as a SPM pixel. An array of SPM pixels into a larger area SPM detector is known as a submodule. A larger array of submodules is known as a module. This is detailed in FIG. 1.
The basis for the SPM is the combination of a Geiger mode photodiode which is biased above the breakdown votlage of the diode and the quenching resistor which is used to quench the photodiode and allow the device to recover and allow detection of another photon. The operation of the SPM operation is described in a publication by Z. Y. Sadygov et al., “Avalanche Semiconductor Radiaton Detectors”, Trans. Nucl. Sci. Vol. 43, No. 3 (1996) pg. 1009-1013. This publication discusses the use of a photodiode which is biased above the breakdown voltage as described by Roland H. Haitz, Model for the electrical behavior of microplasma, Journal of applied physics, vol. 35, no. 5, May 1964, pg. 1370-1376. The use of a resistance to form the passive quenching element required to quench an avalanche breakdown event is discussed in detail in Sadygov. For details of the passive quenching operation, the references by Roland H. Heitz, Studies on Optical Coupling Between Silicon p-n Junctions, Solid-State Electronics, vol. 8, pg. 417-425, 1965 and Robert G. W. Brown and Kevin D. Ridley and John G. Rarity, Characterization of Silicon Avalanche Photodiodes for Photon Correlation Measurements. 1: Passive Quenching, Applied Optics, vol. 25, no. 22, pg. 4122-4126, November, 1986 are recommended. They show the passive quenching of the photodiode which is operated above the breakdown voltage result in a pulse of current flowing through the photodiode during a breakdown event. The full operation of the SPM is detailed in several prior art inventions by the current inventors. These prior art inventions are discussed below:
In our co-pending PCT/GB2006/050123, Light Sensor Module, this patent describes a light sensor module consisting of a number of light sensing elements arranged on a substrate. The module can be operated in a way that produces a combined output signal indicative of an overall level of light falling on the elements. Adjacent light sensing elements sit closely together to form a close-tiled arrangement of the elements covering a large area. This allows a larger area detector to be formed through the use of many smaller detectors combined together.
In our co-pending GB 0621495.1, Method of Assembling a Light Element Module and Light Element Module Assembly, we describe a method of assembling a light sensor module with a light sensing element optically coupled to another optical element, where an intermediate layer is adapted to provide a predetermined level of optical coupling between the optical element and the light sensing element. This allows the formation of a large area arrays of SPM detectors which use a optical layer to provide both electrical connections and the optical coupling to the optical element being viewed or imaged.
In our co-pending GB 0704206.2, Optical Position Sensitive Detector, we describe an optical position sensitive detector which has multiple photosensitive areas, each photosensitive area is capable of producing a signal in response to a photon incident thereon in Geiger mode. Each photosensitive area can provide a signal or signals which may be used to indicate the position of an optical beam incident on the detector surface. This uses a pixellated SPM output which allows for a position sensitive detector to be formed with a high internal gain.
In our co-pending GB 0714770.5, Light Sensor, we describe a method of producing a light sensing arrangement for use in a light sensor. The patent describes how a multiple of individual light sensing elements on a carrier each have a notch formed in them. The purpose of the notch is to allow for electrical connection between the carrier and a surface of the element when the elements are tiled together to form a customisable large detection area.
These allow for the formation of a large area SPM array which is then used to image or to detect light incident onto the array. An issue with the formation of large area detectors however, is the preservation of the timing response of the detectors when they are placed into a large area array. This is particularly important for areas such as high time resolution positron emission tomography, or PET. See reference, Prospects for Time-of-Flight PET using LSO Scintillator, W. W. Moses, and S. E. Derenzo, IEEE Transactions on Nuclear Science NS-46, pp. 474-478 (1999). This system describes a PET system in which the timing properties of lutetium orthosilicate LSO crystals are excited with a 511 keV photon. In a PET system, two 511 keV photons are emitted from the body which are approximately 180 degrees out of phase with one another. The single high energy photon is converted to a number of lower energy photons in the visible spectrum which are detected using a standard PMT in the current state of the art. For LSO approximately 20,000 to 30,000 lower energy visible wavelength photons are emitted for each of the 511 keV photons incident on the crystal. The decay time of the LSO output is approximately 40 ns (See Moses). The challenge for the detection system in a PET system is two fold: First the detectors must be of sufficient high detection efficiency to be able to convert the photons into a measurable response and the detection area is large enough to allow sufficient photons to be obtained. Second the detector must be sufficiently fast to allow the detection of the incident optical pulse with sufficient accuracy to allow the pulse to be measured and analysed. This results in two specifications for PET that must be meet with a detector. These are that the detector must be large area and also fast. As described in Moses, the coincidence resolving time is the time resolution that is obtained from detecting the coincidence between the two photons emitted during a positron annihilation in PET. Because of the speed of light which is a constant given by c=3×10^8 m/s the detector, including both the scintallator crystal and the optical detector must be suitably fast to allow detection of the pulses under with a resolution under 500 ps. This fast resolution is required to allow position of the photon emission to be resolved to a 7.5 cm resolution(see Moses). To increase the resolution, faster detectors are required. It is possible to increase the resolution through repetitive measurements and signal processing, but this slows the data acquisition process and decreases overall system performance. High timing resolution detectors are therefore a requirement for PET. To perform this coincidence timing requires both a fast scintillator crystal, for high energy to lower energy conversion, and a subsequently fast and accurate optical detector with the ability to reconstruct the output pulse from the crystal.
To describe a problem with the current state of the art as appreciated by the present applicant, it is best to review several publications of the inventors and of other state of the art material in the literature. SPM detectors have been shown to have a fast rise time. See the publication by the inventors, Study of the Properties of New SPM Detectors, A G Stewart, E Greene-O'Sullivan, D J Herbert, V Saveliev, F Quinlan, L Wall, P J Hughes, A Mathewson and J C Jackson, SPIE: Semiconductor Photodetectors III, Vol. 6119, 2006. This describes a SPM detector with a 1 mm×1 mm active area with onset times of 1.4 ns and recovery times of 8.7 ns. This active area is too small for many applications requiring large area detection. The inventions of the authors, described in relation to the above-mentioned co-pending applications, were used to overcome the limitations of the smaller area and the subsequent publication, Tiled Silicon Photomultipliers for large area, low light sensing applications, P J Hughes, D Herbert, A Stewart, J C Jackson, Proc. of SPIE: Semiconductor Photodetectors IV, Vol. 6471, 2007 demonstrates the formation of a large area detector with area of 1.2 cm×1.2 cm. This represents a device structure which is 144 times larger than the previous generation of 1 mm×1 mm active area detector. However, as reported in the publication from 2007, the onset response times were on the order of 10 s of nanoseconds. Further analysis and work on this array structure show that the recovery times of the array were degraded significantly over the recovery time of a single SPM pixel. The recovery timing for the array appeared to increase with increasing number of active elements in the array. This results from reducing this to practice show that the output response times are too slow for the recovery of the fast response times that occur in a fast crystal such as LSO, as an example. This is shown graphically in FIG. 2 in which a fast optical source such as a light emitting diode is pulsed via an electrical stimulus. The electrical stimulus is shown in the Figure. This electrical stimulus causes the light source to emit a brief optical pulse which is detected by the SPM submodule. In this Figure we show the response times measured for the SPM submodule of quantity 8 of 3 mm×3 mm SPM pixels. The recovery time for a single SPM 3 mm×3 mm detector was on the order of 50 ns. When the SPM was placed into an array with 8 active elements, the recovery timing degraded to 400 ns which is a factor of 8 times degradation in the output response.
This is unsuitable for high time resolution and fast detection applications such as PET. In patent application WO06111883A2: DIGITAL SILICON PHOTOMULTIPLIER FOR TOF-PET and WO06111869A2: PET/MR SCANNER WITH TIME-OF-FLIGHT CAPABILITY the need for high time resolution detection is discussed. In these patent applications the need for high resolution detectors in a PET or PET/MRI system are described. These applications make use of PCT/GB2006/050123, PCT/GB2006/050122 Digital Avalanche Photodiode, WO04102680A1 A Photodiode, and other patents referenced in this invention claim.
An understanding of the problem was obtained by the present applicant after rigorous analysis of the SPM internal operation and the circuitry which is used to drive and measure the current through the SPM during operation of the array.